Introduction: A Prototype Cargo Trike
I started working with a group that wanted to move some boxes of veggies around town with pedal power. We searched all over the Web for ideas about what to build and ended up with the general plan you see here.
We had access to lots of donor bike parts from a local alternative transportation group. Follow the link below to see how you can get similar parts.
This Ible describes the prototype for the cargo trike that followed. I am working on an Ible describing the next version of the cargo trike we have built. I will add a link to that Ible when I get it published.
Prototype Cargo Trike Specifications:
Overall weight 75#
Cargo portion with front wheels 48#
Rear portion with rear wheel 27#
Cargo Bed 30" X 48"
Pivot Pin 24" from front
Front wheel axles 27" from front
Handlebar is 30" above the bed
Step 1: Selecting the Rear Power Wheel
We were looking for a 26" rear triangle, rear wheel, brake and derailleur. We selected one with a single-piece crank for the prototype. Photo #1 shows the donor bike before we cut it up. Photo #2 shows the pieces after the cuts. We just severed the top and bottom tubes where they met the seat tube. Photo #3 is the part we wanted for the trike power wheel.
Step 2: Designing the Pivot Pin
The pivot pin is where the rear frame main tube hooks to the cargo bed. This is a key point in the design of the cargo trike. It needs to be strong enough to handle the cargo load and it needs to be easy to turn.
We decided to keep things simple in the prototype. For starters the pivot point would be kept vertical - there wouldn't be any fore-or-aft pitch to make the trike lean in turns. This is simpler to build, but it means the rider has to lean into (toward) the turn to help keep the trike upright.
Next we decided to use off-the-shelf parts for the pivot in the prototype. We purchased two 1/2" gate hinges and a 1/2" X 6" bolt at Tractor Supply Company (TSC). Photo #1 shows the hinges and bolt before welding.
Step 3: The Cargo Bed
The cargo bed was 30" by 48" made out of 1" EMT. The two 24" front wheels are mounted in forks taken from two donor bikes. The front forks would normally have been welded to the cargo bed, but in the prototype we experimented with a torsion-bar suspension (that was not particularly useful here).
We decided to build the cargo frame out of EMT conduit even though there is some health issues when welding EMT. Follow the link below for some safety tips.
Step 4: Rear Fork and Linking Tube
The rear fork is connected to the pivot pin via a 40"-long linking tube (Photo #1). The linking tube is composed of two pieces of 1" EMT welded together with two side pieces of 1/2" EMT. This is welded to the lower bracket shell of the rear fork (Photo #2).
This proved to be one of the weak points of the prototype because the linking tube was not strong enough to prevent flexing and torsion along its length. There is a lot of leverage from the seat tube and saddle along the length of this part of the main frame and we still don't have a good design for this part.
Photo #4 shows the "foot brake" on the linking tube and the shift lever located on the seat post.
Step 5: The Handlebar
The handlebar was originally a single bent piece of 1/2" EMT. Photo #1 shows the shape we started with.
This was eventually cut off and replaced by what is shown in Photo #2. The width (30") is fixed, but the height (30") is adjustable. The top part is 1/2" tubing which slides into two vertical 3/4" tubes welded to the cargo bed. A couple of pins hold the handlebar tubing in place.
Step 6: Performance Review
The main frame tube flexes too much. A stiffer design is needed.
The power wheel and seat worked fine. The cart was easy to ride. You do need to lean into the turns or the trike will tend to tip outward. The tipping could be more of a problem if the load had a high center of gravity.
the handlebar was in way when standing up to pedal up a hill. The next design will have two separate hand grips to either side.
The pivot pin and gate hinges worked OK. A stronger cross piece on the cargo bed would prevent some of the side-to-side flexing. Commercial cargo trikes usually have reinforcing tubes that stabilize the lower end of the pivot.
All things considered this was a fun first cargo trike from which we learned a lot. (Note: The cargo bed and wheels are now serving very nicely as a garden cart.)